The present invention is directed to a magnetic shielding arrangement for a mobile MRI system which is mounted in a transportable housing, such as a trailer or prefabricated structure.
Magnetic shielding of a trailer for carrying a mobile MRI (magnetic resonance imaging) system must meet the following criteria: it must contain the magnetic field in accordance with applicable regulations for stray field; adherence to the static site shim specifications must be maintained; and the trailer shielding must not compromise the short-term field stability of the MRI system through bulk movement of the shield.
Measurements of field strength for a known MRI system installed in a mobile trailer indicated that the system suffered from field instabilities, resulting from movement of the shielding steel mounted to the walls of the trailer. In particular, temporal effects on the magnetic field have been attributed to movement of the shielding steel caused by differential thermal expansion across the trailer—differences in expansion coefficients between the outer walls and shield steel—and mechanical wind coupling. All such effects can be increased due to twisting of the trailer when mounted on uneven ground. Such effects are known to limit the performance of such mobile MRI systems.
One object of the present invention is therefore to provide magnetic shielding for a trailer, or other deformable housing such as a prefabricated building, for housing an MRI system, which meets all of the above requirements.
Another object of the invention is to provide magnetic shielding for an MRI system based on, and a deformable housing for, an MRI system, with significant improvements over conventional designs.
Still another object of the invention is to provide a transportable MRI system and housing, in which the MRI system exhibits improved magnetic field stability in the case of deformation of the housing due to the effects of terrain and the environment.
These and other objects and advantages are achieved by the shielding arrangement according to the invention, in which in order to de-couple the shielding from the trailer walls and roof, the trailer is constructed with the shielding mounted to a stiff structure such as a rigid support frame, independent from the outer walls. In addition, in a preferred embodiment of the invention, the arrangement of the steel shielding layers is reversed compared to conventional shielding, in a manner explained hereinafter.
To determine the effectiveness of these measures, the inventors first measured the movement of the magnetic shielding of an MRI system having conventional shielding mounted to the walls of a road trailer, and correlated this movement to the magnetic field drift. The system according to the invention was then tested, and it was shown that the field stability had been improved by a factor of approximately twenty-five over previous measurements (conventional shielding according to the prior art), and was inside the static installation specification for field stability of drift. Measurements indicated that the mobile system had no imaging performance limitations compared to a similar system installed at a static site.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.